Olefins are conventionally polymerized using a Ziegler-Natta catalyst system comprising a procatalyst and a cocatalyst as its essential components. The procatalyst is formed by a transition metal compound of subgroups 4-8 of the periodic system of elements (Hubbard, IUPAC 1970). The cocatalyst is formed by an organic compound of a metal of major groups 1-3 of the periodic system of elements.
The transition metal conventionally is a titanium, zirconium, or vanadium compound, advantageously a titanium compound, and in fact, titanium has been found a particularly advantageous transition metal. Said compounds typically are halides or oxyhalides, or alternatively, organic compounds, conventionally alkoxides, alcoholates or haloalkoxides. Other kinds of organic compounds are less frequently used, while not necessarily unknown in the art. The transition metal compound can be expressed in the form of the following generalized formula: EQU (R'O).sub.n R".sub.m MX.sub.p-n-m (I)
where M is a transition metal of subgroups 4-8, advantageously Ti, Zr or V, while R' and R" represent the similar or dissimilar organic groups chiefly having a backbone of 1-20 carbons, M is a transition metal and X is a halogen, advantageously chlorine. Advantageously and commonly, R' and R" are simply a hydrocarbon group, advantageously an alkyl group. p is the oxidization state of the metal M, commonly p is 4 or 5. n and m are an integer in the range 0-p.
The most advantageous compounds are selected from the group consisting of titanium alkoxides, halides and haloalkoxides, in particular when the halogen is chlorine. Accordingly, suitable compounds include titanium tetramethoxide, tetraethoxide, tetrapropoxides, tetrabutoxides and similar oxides, corresponding titanium alkoxyhalides in which 1-3 alkoxide groups are replaced by a halogen, chlorine in particular, and titanium halides, TiBr.sub.4 and TiCl.sub.4 in particular. The most commonly used of these compounds is TiCl.sub.4. Obviously, two or a greater number of transition metal compounds can be used in the form of different mixtures.
The cocatalyst most commonly consists of an organic compound of a metal of major groups 1-3. While usually an aluminum compound is employed, also boron, zinc and alkali metal compounds have been used. An aluminium compound can be written using the formula (II) as: EQU R.sub.n AlX.sub.3-n (II)
where R is an organic hydrocarbon group, advantageously a C.sub.1 -C.sub.20 alkyl, X is a halogen and n is an integer in the range of 1-3. Different kinds of cocatalyst can be used simultaneously in the form of various mixtures.
Additionally, a catalyst system contains components having catalyst improving and modifying characteristics. The procatalyst can be prepared on a more or less inert support, whereby the procatalyst component may be in solid state even if the transition metal compound as such is not in solid form. The procatalyst can be complexed with a so-called internal donor compound capable of electron donation so as to improve the stereospecificity and/or activity of the catalyst system. The preparation of the procatalyst can be implemented using an auxiliary component which may be a dissolving or slurrying medium and from which a portion is possibly complexed with the procatalyst composition. Such a compound may also act as an electron donor. Also the cocatalyst feed, which typically takes place separately from the procatalyst composition not earlier than to the polymerization process, can be complemented with the electron donor compound with a particular goal of improving the stereospecificity of the end product. Then, the electron donor is called an external donor.
To achieve a heterogenic, solid-state procatalyst composition, a separate support compound is required provided that the transition metal compound of the procatalyst is not one itself. The latter case is true for the transition metal compounds listed above. Widely varied types of solid inorganic or organic compounds can be used as the support. Typical of these are oxides of silicon, aluminium, titanium, magnesium, chromium, thorium or zirconium or mixtures of these oxides, salts of different inorganic acids such as salts of the said metals or earth alkali or earth metals including magnesium silicate and calcium silicate, calcium chloride, calcium sulfate, etc. (cf., e.g., FI patent publication 85,710). Important compounds as supports have been found from magnesium compounds including, e.g., alkoxides, hydroxides, hydroxyhalogenides and halogenides, of which the latter ones, particularly magnesium dichloride is an extremely important support for procatalyst compositions. Supports are typically subjected to different treatments before their use, whereby they can be heat-treated by, e.g., calcining; they can be chemically treated to remove so-called surface hydroxyl groups; they can be mechanically treated by, e.g., grinding in a ball mill or spray mill (cf., e.g., FI Pat. No. 882,626). An important support group is formed by magnesium halides, particularly MgCl.sub.2, which can be advantageously complexed with alcohols, whereby the complexed support can be brought to a morphologically advantageous form by crystallization and/or solidification from an emulsion by a spray-drying technique or from a melt by a spray-crystallization technique (cf., e.g., FI Pat. No. 862,459). Above all, organic supports comprise different polymers either in native form or modified. Of such supports worth mentioning are different polyolefins (polymers made from ethene, propene and other olefins), as well as different polymers of aromatic olefinic compounds (PS, ABS, etc.).
If the olefin monomers being polymerized can assume different spatial configurations when bonding to polymer molecule being formed, this formation generally requires a particular controlling compound capable of complexing the procatalyst so that the new monomer unit being joined to the polymer chain can principally adopt a certain position only. Owing to their manner of bonding to the procatalyst, such compounds are called electron donors, or simply donors. The donor may also render other properties besides the above-mentioned stereospecificity; for instance, the donor may improve the catalyst activity by increasing the bonding rate of the monomer units to the polymer molecule. Such a donor which is incorporated by complexing in the procatalyst already during its preparation is called an internal donor. These donors include a plurality of alcohols, ketones, aldehydes, carboxylic acids, derivatives of carboxylic acids such as esters, anhydrides, halides, as well as different ethers, silanes, siloxanes, etc. Simultaneous use of several donors is also possible. Advantageous compounds in this respect have been found to be, e.g., mono- and diesters of aromatic carboxylic acids and aliphatic alcohols, whose simultaneous use facilitates exchange esterification in conjunction with the use of a donor compound (cf. FI Pat. No. 906,282).
A stereospecificity-controlling compound which is fed into the polymerization reactor only in conjunction with the cocatalyst is called an external donor. Such donors are often the same compounds as those employed as internal donors, while in many cases the external donor in a single polymerization reaction advantageously should not be the same compound as the internal donor, because then the unlike properties of the different compounds can be exploited particularly if the combination of different donors amplifies the effect of their properties and if they have synergistic coeffects. Hence, finding a suitable optimum of such coeffects is the primary goal in the selection of different donors. Advantageous external donors are, e.g., different silane and ether compounds. Particularly alkoxysilanes (cf., e.g., EP Pat. No. 231,878 and EP Pat. No. 261,961) and different linear and cyclic ethers, e.g., trimethyl-methoxyether, dimethoxypropane (cf. EP Pat. No. 449,302) and cineol (cf. Fl Pat. No. 932,580). Also nitrogen-containing heterocyclic compounds have been used such as tetramethylpiperidine (cf. JP Pat. No. 63,105,007).
During the polymerization process, the number of monomer units joining to a polymer molecule may vary from a few units to millions of units. Conventionally, the molecular weight of a commercial-grade solid polyolefin is in the range of 10,000-1,000,000 g/mol. If the degree of polymerization remains lower, the product is a soft and plastic wax or paste-like plastisol, even a viscous liquid which may find use in special applications. A degree of polymerization exceeding one million is difficult to attain, and moreover, such a polymer often is too hard for most applications or too difficult to process. Thus, the molecular weight control of the polymer has an important role, which can be accomplished by means of so-called chain-length controlling agents. Conventionally, the chain-controlling agent added to the polymerization reaction is hydrogen whose benefit is not to introduce any undesirable group in the molecule. If the hydrogen addition is capable of controlling the molecular weight of the produced polymer, the polymerization catalyst is said to be hydrogen sensitive. Different catalyst systems also have different hydrogen sensitivities, whereby different amounts of hydrogen will be required to polymers having the same melt flow rate. On the other hand, hydrogen addition elevates the polymerization activity of the catalyst.
Polymerization can be carried out in gas phase, whereby either gaseous monomer or an inert gas or a mixture thereof is fed to the reactor so that the entering gas keeps the growing polymer in the form of particles on which the growth of the polymer molecules takes place. The reaction temperature is so high that even the monomers are vaporized that are liquid under normal conditions. In a continuous polymerization process the polymer particles are removed continuously from the reactor, and the monomer or monomer mixture feed is continuous. Alternatively, the reaction products removal and precursor feed may also be intermittent. The polymer particle layer which advantageously is kept in a fluidized state can be stirred by mechanical agitation. A great number of different agitator means and agitation systems are available. Gas-phase polymerization is often carried out in a circulating fluidized-bed reactor in which the solid particles form a bed maintained in fluidized state by the upward directed flow of the gaseous feed medium. The fluidized bed may also be formed by inert solids comprised of most varied inorganic and organic compounds.
If liquid-phase polymerization is desired, a medium is required that is liquid at the polymerization temperature, whereby said medium may comprise a single polymer or a greater number of polymers (usually referred to as bulk polymerization), or a separate solvent or diluting agent capable of dissolving or slurrying the monomer or/and the polymer. Where such slurrying involves the formation of a suspension or a slurry, the polymerization process is named respectively. Here, the medium may then be a hydrocarbon solvent particularly including alkanes and cycloalkanes such as propane, butane, isobutane, pentane, hexane, heptane, cyclohexane, etc., which are commonly used. The formation and stability of the slurry may be improved by mechanical agitation, whereby suspending agents called suspenders and colloid stabilizing agents must often be added. The polymerization reactor may be a conventional mixing vessel reactor complemented with widely varying additional arrangements, or alternatively, a loop- or ring-type tubular reactor in which the polymer slurry is circulated by means of different feed, end product removal and agitating arrangements. When the polymerization is carried out in a medium to produce a polymer with a high MFR, catalyst systems of low hydrogen sensitivity may involve problems in the addition of required amount of hydrogen, because only a certain maximum concentration of oxygen can be dissolved in the medium.
Molecular weight distribution (MWD) of polyolefin produced using a high yield Ziegler-Natta catalyst system comprising a titanium-compound-containing procatalyst and an organoaluminium cocatalyst is typically relatively narrow. Polydispersity (Mw/Mn) of polypropylene produced by continuous polymerization reactor with the above mentioned catalyst system is typically about 4-5. When the MWD is broadened, e.g., increasing the polydispersity to 6-8, stiffness of polypropylene can be increased. However, simultaneously impact strength usually decreases. In addition to effect on mechanical properties, broadness of MWD influences on the processability of the polymer. When the increase in stiffness and better processability obtained with fabrication processes using extrusion techniques are combined, it is possible to produce products with markedly higher output and at least similar stiffness.
The broadness of MWD is measured most usually by gel permeation chromatography (GPC), which gives the polydispersity (Mw/Mn). Another relatively common method is based on the effects of MWD on rheological properties. Measuring the shear thinning or elasticity of molten polymer samples, e.g. Shear Thinning Index (SHII) or Elasticity Index, gives good information on broadness of MWD. Broadened MWD increases usually both elasticity and shear thinning of polymer melts.
The most common industrial method to broaden the MWD of polyolefins in continuous polymerization reactors is to produce the polymer in two reactors connected in series, and the polymers produced in each reactor should have clearly different molecular weight. However, it is not always possible or feasible to use two reactors in series to broaden the MWD. According to patent application EP 452916 Al (Idemitsu Petrochemicals KK) MWD of polypropylene can be broadened by using a special type of alkoxysilane as an external donor. In a patent application of Mitsui Petrochemical Ind. Ltd., EP 385765 A2 the MDW of polypropylene is broadened by using a catalyst system having a 1:1 mixture of two different alkoxysilanes as an external donor.